Structures built during wartime sometimes serve as haunting reminders of an era citizens would rather forget. Some of these buildings are demolished at the end of conflict, however others are <a href="http://inhabitat.com/tag/adaptive-reuse/">adapted and given a new life</a>. <a href="http://en.wikipedia.org/wiki/Flak_tower">Flak Towers</a> - anti-aircraft gun shelters originally built to protect the Third Reich - pose this issue, but one tower in the heart of <a href="http://en.wikipedia.org/wiki/Vienna">Vienna</a>, Austria has been transformed into a massive zoo and aquarium called the <a href="http://www.haus-des-meeres.at">Haus des Meeres</a>.

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Haus des Meeres, image by Mr History

The blockhouse towers - enormous concrete and steel leviathans that dominated the skyline - were constructed in Berlin, Hamburg, and Vienna.

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Haus des Meeres, image by Mikrotom

They were used in defense against Allied air raids.

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Flak Towers, image by Julius!

Each one was built over the course of just six months, however they're remarkably durable and feature three and a half meter-thick walls that served as air-raid shelters and sites the <a href="http://en.wikipedia.org/wiki/Luftwaffe">Luftwaffe</a> used to defend the cities.

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Flak Towers, image by Julius!

Located in Esterhazy Park, the site has views over the city, and the building's interior offers five floors of displays.

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Flak Towers, image by Julius!

A multi-story greenhouse has been constructed on the outside of the flak tower, complete with wooden walkways and rope bridges, and tropical vegetation that thrives amid small water features.

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Flak Towers, image by Julius!

The Haus des Meeres also has a vivarium where visitors can find snakes, lizards, and crocodiles, and even a shark aquarium on the ground floor.

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Haus des Meeres, image by Sebastian Neidlich (Grabthar)

Structures built during wartime sometimes serve as haunting reminders of an era citizens would rather forget. Some of these buildings are demolished at the end of conflict, however others are adapted and given a new life. Flak Towers - anti-aircraft gun shelters originally built to protect the Third Reich - pose this issue, but one tower in the heart of Vienna, Austria has been transformed into a massive zoo and aquarium called the Haus des Meeres.